During the last decade, the automotive and aircraft industries have concentrated their efforts on reducing the overall weight of the vehicles and aircraft. Initially, weight reduction was achieved by replacing metal parts with suitable plastic parts. However, in structural applications, the plastic parts are very often lacking in mechanical performance. Thus, to improve their performance, various reinforcing fibrous materials, such as glass, carbon or aramid fibers, are added to the plastic resin. As a cost savings measure, inorganic fillers, such as talc, calcium carbonate or clay are also added to the resin.
However, the addition of the fibers and inorganic fillers undesirably increases the composite's specific gravity and weight per molded part. Additionally, an increase in price per unit volume may follow. To counteract this weight increase, it is known that hollow glass spheres may be added. Traditionally, the host matrix for the hollow spheres has been a thermosetting resin which does not present the processing problems of thermoplastic resins. Before curing, thermoset resins have a relatively lower viscosity than thermoplastic resin and thereby provide a more readily workable medium. Generally speaking, the more viscous thermoplastic resins require more work and higher shear forces in order to process before cure. Consequently, it is more difficult to compound fragile additives into thermoplastics without substantial breakage.
To minimize fracture when compounding into thermoplastic resins, relatively strong glass spheres can be used, as disclosed in U.S. Pat. No. 4,391,646. However, when combined with relatively high amounts of reinforcing glass fiber, the inventors have found that sphere fracture is intensified. It is believed that the abrasive physical contact between the rigid hollow glass spheres and the fiberglass, under high shear forces, causes additional sphere fracture. Even further fracturing of the spheres is known to occur during subsequent compression and, in particular, injection molding processes, used to form objects, such as car bumpers.
Hollow sphere fracture is undesirable because it raises the specific gravity of the composite and defeats the purpose for adding the spheres. In order to take full advantage of the specific gravity reduction seen in the presence of hollow spheres, the integrity of the spheres must be maintained during compounding and subsequent processing or molding.
The inventors have also observed that the hollow glass spheres similarly degrade glass fiber length and that further processing dramatically diminishes the fiber length below that necessary for efficient stress transfer from the polymer matrix to the fiber. Fibers which are shorter than this critical length will be less effective as reinforcements. In order to improve the mechanical properties of glass fiber thermoplastic polyurethane composites, the fiber length must be maximized.
A method has now been discovered which diminishes or eliminates fracture of hollow glass spheres, and degradation of glass fiber length, in thermoplastic composites made by melt compounding, thus lowering and maintaining the lowered specific gravity of the composite.
This invention embodies the discovery that hollow glass sphere fracture is minimized in fiberglass reinforced thermoplastic resin composites made by melt compounding, by adding a concentrate of spheres dispersed in thermoplastic resin (masterbatch). Superior results will be realized in some instances, i.e., fracture can be eliminated, when the masterbatch is made by adding the spheres during the in situ polymerization of the masterbatch resin. It is believed that the masterbatch resin covering the glass spheres protects them from breakage by lessening the effects of the shear forces exerted by the mechanical action of the compounding apparatus and the abrasive contact with the glass fibers. Consequently, by substantially maintaining the integrity of the spheres, one is able to maintain the lowered specific gravity of the composite. It has been further discovered that the inventive thermoplastic composites continue to minimize sphere fracture even when further processed in high stress molding methods.